Metal-air batteries are comprised of multiple electrochemical cells. Each cell is further comprised of an air permeable cathode and a metallic anode separated by an aqueous electrolyte. Metal-air batteries have a relatively high energy density because they utilize oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal oxide or other depolarizable metallic composition. For example, during discharge of a zinc-air battery cell, oxygen from ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode, reacts with the hydroxide ions, and water and electrons are released to provide electrical energy.
The anodes are made from metals which can be oxidized during discharge in a metal-air cell to produce electrical energy. Such metals include lead, zinc, iron, cadmium, aluminum and magnesium. Zinc is normally preferred because of the availability, energy density, safety, and relatively low cost of zinc.
A suitable electrolyte is an aqueous electrolyte including group I metal hydroxides such as LiOH, NaOH, KOH, CsOH, or the like.
Battery cells that are used for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through the air permeable cathode and the anode is electrolytically reformed by reducing to the base metal the metal oxides formed during discharge.
Metal-air battery cells are often arranged in multiple cell battery packs within a common casing to provide a sufficient amount of power output. The casing is necessary to seal-off the cells from the ambient air to prevent self discharge of the cells during periods of non-use, which would result in a decreased battery output and life time. When in use, small ventilation holes in the housing are open to allow for ambient air to flow across the air cathodes of the metal-air cells.
Metal-air batteries provide a relatively light weight and compact power supply because they use oxygen from ambient air as a reactant in the electrochemical reaction. Due to the light weight and compactness of metal-air batteries, they are an ideal source of power for portable equipment.
Most electrochemical batteries use corrosive electrolytes and are susceptible to damage and leakage. Metal-air batteries are especially so due to their use of oxygen from the ambient air as a reactant. Because the air electrode in a metal-air cell is exposed to the outside environment, flooding of the electrode can cause the cell to leak. Additionally, if oxygen is not properly exhausted to the atmosphere during recharge of a secondary metal-air battery cell, oxygen pressure inside the cell may cause the cell to leak due to a breach of the air electrode structure.
Cell leakage is a potential problem in many batteries including metal-air batteries because they use caustic electrolytes which are corrosive in their aqueous form. Therefore, it would be beneficial to be able to determine whether the cells of a metal-air battery are leaking electrolyte so that the battery can be removed from service to reduce the risk of damage to the equipment being used in conjunction with the battery.
However, because metal-air batteries require a sealed casing for efficient use, routine visual inspection of the cells for leakage is not possible. Therefore, there exists a need for a battery leakage detector that immediately informs a user of electrolyte leakage occurring inside the battery casing, gives an indication of that leakage and prevents the leaked electrolyte from further escaping to the outside of the battery casing.
Various structures have been proposed for detecting and indicating battery leakage. For example, U.S. Pat. No. 4,222,745 to Cloyd, Great Britain Patent No. 2,164,200 to Babi, U.S. Pat. No. 684,697 to Lloyd, U.S. Pat. No. 675,708 to Blackwell, and Japanese Patent No. 59-51546 to Shimizu, disclose such detectors. Cloyd proposes a paste-like composition of potassium dichromate absorbed on finely divided silica and a polymeric adhesive material to detect leakage of sulfur dioxide from a battery cell. The composition is applied to vertical and horizontal services of a battery cell or battery casing. Upon leakage of sulfur dioxide into the composition, the composition changes colors from a yellow to a bluish green, indicating that a hazardous condition exists.
The goal of Cloyd appears to be merely to detect and indicate the leakage of sulfur dioxide once the leak has spread to the composition. However, at that time, the hazard of injury or damage from the leaking electrolyte may have already existed for some time. Additionally, the composition does not prevent leaked electrolyte from further escaping outside of the battery casing.
Babi discloses a composition of matter that changes color when a non-aqueous electrolyte leaks onto the composition. Like Cloyd, the composition only indicates an electrolyte leak once the leak has spread to the composition. Also like Cloyd, the composition does not prevent the further escape of the leaking electrolyte.
Lloyd describes a battery draining box which electrically detects battery leakage and activates an electrical alarm. The draining box includes a drain pipe which leads to a conductive bowl and then to a boot. The conductive bowl has a tail which is connected to a device with a spring such that the bowl is held directly in the, middle of the drain pipe. The alarm device has one contact wired to the drain pipe and the other wired through the battery to a tail of the conductive cup. When solution leaks from a battery, the drain box collects the solution and sends it into the drain pipe where it drips into the bowl. When the bowl is sufficiently full, the weight of the bowl overcomes the spring causing the bowl to tilt downwards and contact the side wall of the drain pipe. This completes the electrical circuit and causes the alarm to activate. In another embodiment, Lloyd describes the use of two electrical contacts in the drain pipe. The alarm circuit is completed when leaked electrolyte passes between the contacts.
Lloyd is impractical in requiring a drain box, pipe and boot which add considerable weight and bulk to a battery. Further, Lloyd allows the leaked solution to remain free in an open tray where it can be splashed, spilled, or otherwise escape and cause injury or damage. Additionally, Lloyd requires the battery to remain at a fixed orientation. Thus, it is not a feasible for portable batteries which are transported and used at various orientations.
Blackwell describes an open tray for holding battery cells. The tray bottom is lined with zinc. Carbon elements are electrically wired together to extend across the tray above, but not in contact with, the zinc bottom. An alarm device has one pin wired to a binding post in contact with the zinc bottom and has a second pin wired to a binding post connected to the carbons. When a battery cell resting in the tray leaks a sufficient amount of conductive liquid to complete the electrical circuit between the zinc bottom and the carbon elements suspended above, an electrical alarm is activated.
Blackwell, which uses an open tray similar to Lloyd, is also not feasible for portable batteries. Further, Blackwell uses multiple large electrical contacts which add a relatively large amount of weight and bulk to the battery. Thus, like Lloyd, Blackwell is not practical for portable batteries that must be compact and relatively light weight.
Shimizu describes a vacuum deposited film of Tungsten Oxide (WO.sub.3) which changes from non-conductive to conductive when in contact with an electrolyte. The film is connected to a battery and provides a bridge between electrical contacts. When leaking electrolyte comes in contact with the film, the film creates a conductive path to complete a circuit and activate an alarm.
The goal of Shimizu appears to be merely to detect and indicate that a leak has occurred when a sufficient amount of electrolyte has leaked onto the film between the contacts. Thus, if an electrolyte leak occurs away from the film or away from the contacts no alarm will be activated. Further, Shimizu allows a leaked electrolyte to remain free where it can leak out of the battery casing to cause injury or damage.
Thus, there exists a need for a leak detector capable of detecting and indicating electrolyte leakage which occurs away from the leak detection means, such as electrical contacts. Further, there exist a need for a battery leak detector which will detect leaked electrolyte and prevent the electrolyte from further escaping to damage equipment or injure users.